Optical pickup lens device and information recording and reproducing device using the same

ABSTRACT

An optical pickup lens device includes, in the order from the light source side, collimating means for converting a bundle of rays into parallel rays or predetermined convergent or divergent rays, the collimating means being movably held along a direction of an optical axis of a bundle of rays emitted from a light source; an aberration correcting element for allowing a bundle of rays emitted from the collimating means to be transmitted therethrough; and an objective lens element having a numerical aperture of 0.8 or more, and converging a bundle of rays coming from the aberration correcting element onto the information recording medium to form a spot. The aberration correcting element and the objective lens element are integrally held together in a direction orthogonal to the optical axis so as to perform tracking on the information recording medium, and satisfy predetermined conditions.

TECHNICAL FIELD

The present invention relates to an optical pickup lens device, and inparticular to an optical pickup lens device applicable to high-densityrecordable optical information recording devices such as DVD (DigitalVersatile Disk) devices which use a bundle of rays with wavelengths of390 nm to 420 nm, and optical recording devices for computers. Inaddition, the present invention relates to an information recording andreproducing device having the aforementioned optical pickup lens device.

BACKGROUND ART

Conventionally, an optical pickup lens device is used under conditionsin which the wavelength of a light source is 650 nm or more and thenumerical aperture of an objective lens element is such that NA=on theorder of 0.6; therefore the amount of displacement of a spot caused byaxial chromatic aberration or magnification chromatic aberration is notlikely to cause problems.

In recent years, however, along with an increase in the capacity ofinformation recording media, there have been developments in theshortening of the wavelength of a light source and the increase of NA(numerical aperture) in an optical information recording device. In sucha short-wavelength region, since the dispersion of an optical materialsuch as a lens element is very large, a slight change in the wavelengthof a bundle of rays significantly changes the refractive index of theoptical material. Accordingly, in recent optical pickup lens devices, itis necessary to consider chromatic aberration correction.

Particularly, in an optical information recording device of DVD±RW,etc., which are currently widely used in a DVD recorder, etc., since therecording and erasing of information are performed using a phase changein a medium, the optical power used when writing or erasing informationis made different from the optical power used when reading writteninformation. Thus, in an optical information recording device using aphase-transition medium, it is not possible, in principle, to avoid thewavelengths of a bundle of rays emitted by the light source fromsignificantly changing upon switching between recording or erasing andreproduction.

Hence, in an optical information recording device using aphase-transition medium, chromatic aberration correction in an opticalpickup lens device is a critical problem for the following reason. In anoptical pickup device using a phase-transition medium, if the chromaticaberration in the lens device is not corrected, a steep focus positionchange occurs due to the change in wavelength emitted by the lightsource, and consequently, focus control may not be performed.

Conventionally, in order to suppress such chromatic aberration, asdescribed in Japanese Laid-Open Patent Publications No. 64-19316, No.7-294707, and No. 11-337818, there have been proposed techniques such asa technique of allowing an objective lens element to have chromaticaberration correction functionality, a technique of allowing acollimating lens disposed between a light source and an objective lenselement to have chromatic aberration correction functionality, atechnique of additionally inserting a chromatic aberration correctingelement in an optical path to excessively correct chromatic aberration,and thereby canceling out chromatic aberration in an objective lenselement.

DISCLOSURE OF THE INVENTION

However, conventional structures which realize chromatic aberrationcorrection are not satisfactory for an optical pickup lens device whichuses a high-density recordable information recording medium in which thespot diameter is very small and the track width is very narrow.

An object of the present invention is to provide an optical pickup lensdevice capable of performing stable tracking while having largechromatic aberration correction functionality, and an informationrecording and reproducing device using the optical pickup lens device.

One of the aforementioned objects is achieved by the following opticalpickup lens device. An optical pickup lens device used in an opticalpickup device which performs at least one of reading, writing, anderasing of information by converging onto an information recordingmedium a bundle of rays with wavelengths of from 390 nm to 420 nmemitted from a light source, to form a spot, comprises, in an order froma side of the light source: collimating means for converting the bundleof rays into parallel rays or predetermined convergent or divergentrays, the collimating means being movably held along a direction of anoptical axis of the bundle of rays emitted from the light source; anaberration correcting element for allowing a bundle of rays emitted fromthe collimating means to be transmitted therethrough; and an objectivelens element having a numerical aperture of 0.8 or more, and converginga bundle of rays coming from the aberration correcting element onto theinformation recording medium to form a spot, wherein the aberrationcorrecting element and the objective lens element are integrally heldtogether in a direction orthogonal to the optical axis so as to performtracking on the information recording medium, and the optical pickuplens device satisfies the following conditions:−0.1≦CAt≦−0.1  (1);−20≦CAf≦20  (2);−20≦CAm≦0  (3);−0.25≦θf≦0.25  (4); and−0.75≦θm≦0.75  (5),

-   -   where    -   CAt: axial chromatic aberration (μm/nm) in an entire optical        system,    -   CAf: axial chromatic aberration (μm/nm) in the collimating        means,    -   CAm: axial chromatic aberration (μm/nm) in the aberration        correcting element,    -   θf: amount of change in an angle of a bundle of outgoing rays        from the collimating means per unit wavelength (min/nm), and    -   θm: amount of change in an angle of a bundle of outgoing rays        from the aberration correcting element per unit wavelength        (min/nm).

Preferably, the aberration correcting element is a diffractive lenshaving an optical power to deflect a bundle of rays by diffraction.Alternatively, the aberration correcting element preferably has a phasestep surface including a plurality of zone regions defined by concentriccircles with the optical axis being at a center; and phase steps eachformed at a boundary portion between the regions.

One of the aforementioned objects is achieved by the following opticalpickup device. An optical pickup device which performs at least one ofreading, writing, and erasing of information by converging a bundle ofrays onto an information recording medium to form a spot, comprises: alight source for emitting a bundle of rays with a wavelength range of390 nm to 420 nm; collimating means for converting the bundle of raysinto parallel rays or predetermined convergent or divergent rays, thecollimating means being movably held along a direction of an opticalaxis of the bundle of rays emitted from the light source; an aberrationcorrecting element for allowing a bundle of rays emitted from thecollimating means to be transmitted therethrough; and an objective lenselement having a numerical aperture of 0.8 or more, and converging abundle of rays coming from the aberration correcting element onto theinformation recording medium to form a spot, wherein the aberrationcorrecting element and the objective lens element are integrally heldtogether in a direction orthogonal to the optical axis so as to performtracking on the information recording medium, and the optical pickupdevice satisfies the following conditions:−0.1≦CAt≦−0.1  (1);−20≦CAf≦20  (2);−20≦CAm≦0  (3);−0.25≦θf≦0.25  (4); and−0.75≦θm≦0.75  (5),

-   -   where    -   CAt: axial chromatic aberration (μm/nm) in an entire optical        system,    -   CAf: axial chromatic aberration (μm/nm) in the collimating        means,    -   CAm: axial chromatic aberration (μm/nm) in the aberration        correcting element,    -   θf: amount of change in an angle of a bundle of outgoing rays        from the collimating means per unit wavelength (min/nm), and    -   θm: amount of change in an angle of a bundle of outgoing rays        from the aberration correcting element per unit wavelength        (min/nm).

According to the present invention, an optical pickup lens devicecapable of performing stable tracking while having large chromaticaberration correction functionality, and an information recording andreproducing device using the optical pickup lens device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic schematic structure diagram of an optical pickupdevice in a first embodiment of the present invention.

FIGS. 2 are diagrams showing aberration in a collimating lens of a firstexample in the first embodiment of the present invention.

FIGS. 3 are diagrams showing aberration in a diffractive lens of thefirst example in the first embodiment of the present invention.

FIGS. 4 are diagrams showing aberration in a collimating lens of acomparative example in the first embodiment of the present invention.

FIGS. 5 are diagrams showing aberration in a diffractive lens of thecomparative example in the first embodiment of the present invention.

FIG. 6 is a schematic structure diagram of the optical pickup device inthe first embodiment of the present invention.

FIG. 7 is a schematic structure diagram of an optical pickup deviceaccording to a second embodiment.

FIG. 8 is a schematic structure diagram showing a lens device used inthe optical pickup device according to the second embodiment.

FIG. 9 is a schematic diagram showing a structure of a phase stepsurface of an aberration correcting element of the lens device used inthe optical pickup device according to the second embodiment.

FIG. 10 is a graph showing spherical aberration in a lens device of asecond numerical example at wavelengths of 410 nm±10 nm.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 is a schematic structure diagram of an optical pickup lens deviceaccording to a first embodiment of the present invention. The opticalpickup lens device according to the first embodiment includes a lightsource 1, a collimating lens 3 (collimating means), a diffractive lens 4(an aberration correcting element), an objective lens 5, and an actuator7. The light source 1 is composed of a semiconductor laser and emits abundle of rays 2 with wavelengths ranging from 390 nm to 420n.

In FIG. 1, the bundle of rays 2 coming from the light source 1 composedof a semiconductor laser become substantially parallel rays through thecollimating lens 3. Then, the rays are transmitted through thediffractive lens 4 and then converged onto an information recordingmedium 6 by the objective lens 5.

Here, the diffractive lens 4 along with the objective lens 5 are mountedon the actuator 7 such that their optical axes are substantially alignedwith each other, whereby the diffractive lens 4 and the objective lens 5are movable in directions perpendicular to the optical axis direction,as shown by arrows A and −A. Accordingly, even if the wavelength ischanged and a bundle of rays diverge or converge, compensation fordisplacement of a spot in a track direction, i.e., tracking, can beperformed.

The collimating lens 3 is composed of a cemented achromatic lens and ismovable in the optical axis direction, as shown by arrow B, so thatspherical aberration occurring in an optical system can be corrected. Bythis, the angle of a bundle of rays entering the objective lens 5 can bechanged, whereby spherical aberration caused due to the difference inthe thickness of the information recording medium 6 or due to eachoptical element composing the optical system, can be canceled out.

The optical pickup lens device according to the first embodimentsatisfies the following conditions:−0.1≦CAt≦−0.1  (1);−20≦CAf≦20  (2);−20≦CAm≦0  (3);−0.25≦θf≦0.25  (4); and−0.75≦θm≦0.75  (5),

-   -   where    -   CAt: axial chromatic aberration in the entire optical system        (μm/nm),    -   CAf: axial chromatic aberration in the collimating means        (μm/nm),    -   CAm: axial chromatic aberration in the aberration correcting        element (μm/nm),    -   θf: amount of change in the angle of a bundle of outgoing rays        from the collimating means per unit wavelength (min/nm), and    -   θm: amount of change in the angle of a bundle of outgoing rays        from the aberration correcting element per unit wavelength        (min/nm).

If CAt, the axial chromatic aberration in the entire optical system, issmaller than −0.1 (μm/nm) or greater than 0.1 (μm/nm), because theamount of movement of the spot in the optical axis direction caused by awavelength change is great, it is difficult to perform stable recordingor reproduction and thus it is not desirable.

If the axial chromatic aberration in the collimating means CAf issmaller than −20 (μm/nm), when the axial chromatic aberration in theaberration correcting element CAm is 0, the amount of displacement ofthe spot exceeds 10 nm, and thus it is not desirable. If the axialchromatic aberration in the collimating means CAf is greater than 20(μm/nm), it becomes difficult to satisfy the aforementioned condition(1) with the aberration correcting element, and thus it is notdesirable.

If the axial chromatic aberration in the aberration correcting elementCAm is smaller than −20 (μm/nm), even if the collimating means is inchromatic aberration under correction condition, the aforementionedexpression (1) cannot be satisfied and thus it is not desirable. If theaxial chromatic aberration in the aberration correcting element CAm isgreater than 0, chromatic aberration in the objective lens elementcannot be corrected and thus it is not desirable.

If the amount of change in the angle of a bundle of outgoing rays fromthe collimating means per unit wavelength Of is smaller than −0.25(min/nm) or greater than 0.25 (min/nm), even if chromatic aberration inthe aberration correcting element is 0, the amount of displacement ofthe spot exceeds 10 nm, and thus it is not desirable.

If the amount of change in the angle of a bundle of outgoing rays fromthe aberration correcting element per unit wavelength θm is smaller than−0.75 (min/nm) or greater than 0.75 (min/nm), the magnification of theoptical system greatly varies from the numerical aperture NA of theobjective lens element, and thus it is not desirable in terms of thestructure of the optical pickup device.

FIG. 6 is a schematic structure diagram of an optical pickup device towhich the optical pickup lens device according to the first embodimentof the present invention is applied. In FIG. 6, the same components asthose in FIG. 1 are designated by the same reference numerals. In FIG.6, a bundle of rays coming from a light source 1 composed of asemiconductor laser are transmitted through a beam splitter 8 and thenbecome substantially parallel rays through a collimating means 3composed of a collimating lens. Then, the rays are transmitted throughan aberration correcting element 4 composed of a diffractive lens, andthen converged onto an information recording surface 6 a of aninformation recording medium 6 by an objective lens 5. A convergencespot where the bundle of rays are converged onto the informationrecording surface 6 a is reflected by pits formed on the informationrecording surface 6 a and having different reflectivities, and thereflected laser light is transmitted through the objective lens 5, theaberration correcting element 4, and the collimating means 3, andreflects by the beam splitter 8, and refracts at a detection lens 9, andthen is converged onto a light receiving element 10. Using an electricalsignal from the light receiving element 10, the change in the amount oflight modulated at the information recording surface 6 a is detected,and data recorded on the information recording medium 6 is read.

Here, the aberration correcting element 4 and the objective lens 5 areboth mounted on an actuator 7 and a removable in directions of arrows Aand −A, i.e., in a direction orthogonal to the optical axis direction,and the collimating lens 3 is movable in the optical axis direction, asshown by arrow B.

Note that, in the first embodiment, the collimating lens is composed ofa cemented lens, but may be composed of a diffractive lens with colorcorrection functionality or a single lens with no color correctionfunctionality. In addition, the aberration correcting element iscomposed of a diffractive lens, but may be composed of a cemented lenswith chromatic aberration correction functionality. However, since adiffractive lens can be formed of a resin and thus is light in weight,it is advantageous to mount a diffractive lens along with an objectivelens on an actuator and allow them to move.

Note also that the aberration correcting element, i.e., a diffractivelens, and the objective lens are configured separately, but may beconfigured integrally such that at least one surface of the objectivelens has a diffraction structure.

FIRST NUMERICAL EXAMPLE

Now, a numerical example which concretely shows the optical pickup lensdevice according to the first embodiment is described along with acomparative example. A first example and the comparative example aredifferent only in the design values of a collimating lens 3 and adiffractive lens 4. In the first example and the comparative example,although both examples have the same axial chromatic aberration in theentire optical system of 0.09 μm/nm, axial chromatic aberration and theamount of change in outgoing angle for the collimating lens 3 aredifferent from those for the diffractive lens 4, and thus the amount ofdisplacement of the spot is significantly different for each example.

A specific numerical structure of a collimating lens 3, a diffractivelens 4, and an objective lens 5 of the first example is shown in Table1, and similarly, a numerical structure of the comparative example isshown in Table 2. In each example, the center design wavelength is 410nm. In addition, in the first example and the comparative example,parallel beams are assumed to enter the diffractive lens 4, and thediameter of the parallel beams on the outgoing side is set to 2.21 mm.Surface numbers 1 to 4 represent the collimating lens 3, surface numbers5 to 8 represent the diffractive lens 4, surface numbers 9 and 10represent the objective lens 5, and surface numbers 11 and 12 representa protective layer of an information recording medium 6 which is amedium. Note that r represents the radius of curvature of each lenssurface (a protective layer surface for the information recordingmedium), d represents the lens thickness, nλ represents the refractiveindex of each lens at a wavelength of λ nm, and ν represents the Abbeconstant of each lens. A phase grating formed on a diffractive surfaceis represented by an ultra-high refractive index method (for theultra-high refractive index method, see William C. Sweatt: Describingholographic optical elements as lenses: Journal of Optical Society ofAmerica, Vol. 67, No. 6, June 1977).

The aspherical shape is given by the following (Eq. 1): $\begin{matrix}{X = {\frac{C_{j}h^{2}}{1 + \sqrt{1 - {\left( {1 + k_{j}} \right)C_{j}^{2}h^{2}}}} + {\Sigma\quad A_{j,n}h^{n}}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$

The meaning of each symbol is as follows:

-   -   X: distance of a point on an aspherical surface whose height        from the optical axis is h, from a plane tangent to the vertex        of the aspherical surface,    -   h: height from the optical axis,    -   Cj: curvature of the j-th surface of the objective lens at the        vertex of the aspherical surface (Cj=1/Rj),    -   Kj: conic constant of the j-th surface of the objective lens,        and    -   Aj, n: n-th order aspherical coefficient of the j-th surface of        the objective lens,

where j=13, 14. TABLE 1 Surface no. r d n410 νd Object point 1 — 6.114 2Plane 4.250 1.52957 64.2 3 Plane 1.100 1.56124 69.5 4 — 5.793Collimating 5 84.725 0.500 1.73959 30.1 6 12.469 0.000 7 12.469 1.0001.68490 55.4 8 −12.037 5.207 Diffractive 9 75113.020 0.000 4101.03141−3.5 10 Plane 0.000 11 Plane 0.500 1.52256 56.4 12 9.402 2.000 Objective13 1.097 1.907 1.77717 45.6 14 −3.126 0.252 Disk 15 Plane 0.100 1.6158030.1 16 Plane K13 −0.843174 A13, 4 0.034142332 A13, 6 0.036644763 A13, 8−0.09167153 A13, 10 0.14483359 A13, 12 −0.07839237 A13, 14 −0.01510398A13, 16 0.017046335 K14 33.80017 A14, 4 0.25550467 A14, 6 14.441438 A14,8 −164.31079 A14, 10 722.07909 A14, 12 −732.06737 A14, 14 −3351.5353A14, 16 7740.7264

TABLE 2 Surface no. r d n410 νd Object point 1 — 6.036 2 Plane 4.25 1.52957 64.2 3 Plane 1.1  1.56124 69.5 4 — 5.773 Collimating 5   42.8910.500 1.73959 30.1 6    2.200 0.000 7    2.200 1.000 1.68490 55.4 8  −10.000 5.227 Diffractive 9 191418.600 0.000 4101.03141 −3.45 10   0.000 0.000 11    0.000 0.500 1.52256 56.4 12   24.225 2.000Objective 13    1.097 1.907 1.77717 45.6 14   −3.126 0.252 Disk 15 Plane0.100 1.61580 30.1 16 Plane K13 −0.843174 A13, 4 0.034142332 A13, 60.036644763 A13, 8 −0.09167153 A13, 10 0.14483359 A13, 12 −0.07839237A13, 14 −0.01510398 A13, 16 0.017046335 K14 33.80017 A14, 4 0.25550467A14, 6 14.441438 A14, 8 −164.31079 A14, 10 722.07909 A14, 12 −732.06737A14, 14 −3351.5353 A14, 16 7740.7264

Aberration in the collimating lens 3 of the first example is shown inFIG. 2(a) shows spherical aberration SA and FIG. 2(b) shows the amountof offense against sine condition SC. Further, aberration in thediffractive lens 4 which is an aberration correcting element of thefirst example is shown in FIG. 3. FIG. 3(a) shows spherical aberrationSA and FIG. 3(b) shows the amount of offense against sine condition SC.

As shown in FIG. 2(a), the spherical aberration SA in the collimatinglens 3 is substantially favorably corrected. In addition, as shown inFIG. 2(b), the sine condition SC is also substantially corrected.

Aberration in the collimating lens 3 of the comparative example is shownin FIG. 4. As shown in FIG. 4(a), spherical aberration SA in thecollimating lens 3 is substantially favorably corrected. Similarly, asshown in FIG. 4(b), sine condition SC is also favorably corrected.Aberration in the diffractive lens 4 of the comparative example is shownin FIG. 5. As shown in FIG. 5(a), spherical aberration SA in thediffractive lens 4 is substantially favorably corrected; similarly, asshown in FIG. 5(b), sine condition SC is also favorably corrected.

Table 3 shows the focal length, effective aperture, and axial chromaticaberration for each of the collimating lens 3, the diffractive lens 4,and the objective lens 5, and axial chromatic aberration in the entireoptical system, for the first example and the comparative example. TABLE3 Entire Colli- Diffractive Objective optical mating lens lens systemUnit Focal 16.4 Afocal 1.3 mm length Effective 2.80 3.00 2.21 mmaperture 1st Axial −0.05 −15.66 0.29 0.09 μm/mm ex chromatic aberrationCom. Axial −25.56 −5.99 0.29 0.09 μm/mm ex chromatic aberration

Table 4 shows the amount of change in outgoing angle per light sourceunit wavelength for each of the collimating lens 3 and the diffractivelens 4 for the first example and the comparative example. θ1 representsthe amount of change in the outgoing angle of a bundle of rays per 1-nmchange in the wavelength of the light source for each of the collimatinglens 3 and the diffractive lens 4 when the objective lens 5 and thediffractive lens 4 of the first example are integrally shifted by 150 μmin a track direction of the information recording medium 6. θ2 similarlyrepresents the amount of change in the outgoing angle of a bundle ofrays for the comparative example. TABLE 4 Collimating Diffractive lensUnit θ1 0.001 0.590 min/mm θ2 0.470 0.697 min/mm

Further, Table 5 shows the amount of displacement of the spot in a focaltrack direction per light source wavelength unit change for the firstexample and the comparative example. D1 represents the amount ofdisplacement of a spot position of the information recording medium 6 ina track direction at a focal point per 1-nm change in light sourcewavelength when the objective lens 5 and the diffractive lens 4 of thefirst example are integrally shifted by 150 μm in a disk trackdirection. D2 similarly represents the amount of displacement of a spotposition for the comparative example. TABLE 5 Amount of displacement ofspot Unit D1 0 nm D2 17.5 nm

In the first example, chromatic aberration in the collimating lens 3 issufficiently corrected, and the amount of chromatic aberrationcorrection for the diffractive lens 4 is large and thus a chromaticaberration overcorrection condition exists. On the other hand, in thecomparative example, the collimating lens 3 is large and thus achromatic aberration overcorrection condition exists, and the amount ofchromatic aberration correction for the diffractive lens 4 is smallerthan that for the first example. Thus, it can be seen that, as shown inTable 4, the degree of divergence/convergence of a bundle of outgoingrays from the collimating lens 3 and the diffractive lens 4 increases inproportion to the chromatic aberration. As shown in FIG. 5, even if theoptical pickup devices have the same chromatic aberration in the entireoptical system, the amount of displacement of the spot in the disk trackdirection greatly varies due to the difference in the distribution ofthe amount of chromatic aberration correction. In the first example, theamount of displacement of the spot is 0.0 nm per 1 nm; i.e.,substantially no spot displacement occurs. Therefore, stable recordingand reproduction can be performed. In the comparative example, the spotis abruptly moved by as much as 17.5 nm per 1-nm wavelength, and thusthere is a high risk of causing off-track errors.

It is desirable that the color of the aforementioned collimating lens 3be sufficiently corrected; if color correction is not sufficiently done,substantially parallel rays coming from the collimating lens 3 causedivergence or convergence as a result of the wavelength change. Thisamount of change in angle affects the amount of displacement of the spotif the objective lens 5 is shifted.

Under present circumstances, when the amount of shift of the objectivelens 5 is 150 μm, if the wavelength is changed by 1 nm, the amount ofdisplacement of the spot in the disk track direction at which stabletracking can be performed is considered to be less than 10 nm.

If a color overcorrection condition or a color undercorrection conditionexists, the amount of change in the angle of a bundle of outgoing raysfrom the light source per unit wavelength change becomes large. That is,when a change in the wavelength of the light source occurs, the anglesof the divergence and convergence of a bundle of outgoing rays becomelarge, and even if the diffractive lens 4, which is an aberrationcorrecting element, and the objective lens 5 are shifted in a coaxialstate at all times, because light enters the moving part of thediffractive lens 4 and the objective lens 5 from off-axis points, theamount of movement of the spot at a focal point in the track directionbecomes large.

According to the first example, it is possible to correct large axialchromatic aberration which occurs as a result of an abrupt change in thewavelength of the light source in a short wavelength region. Inaddition, even if the aforementioned wavelength change occurs when theoptical axis is shifted in the track direction of an informationrecording medium because of the objective lens performing tracking, itis possible to suppress the amount of movement of the spot at a focalpoint in the track direction of the information recording medium. Thatis, the first example has a large amount of axial chromatic aberrationcorrection and a large amount of magnification chromatic aberrationcorrection. On the other hand, although the comparative example has thesame amount of axial chromatic aberration correction as the firstexample, the amount of magnification chromatic aberration correction issmall.

As described above, according to the optical pickup lens device of thefirst embodiment, by appropriately distributing the amount of chromaticaberration correction for each of a collimating means and an aberrationcorrecting element, even if the wavelength of the light source isquickly changed when the objective lens is shifted from the optical axisbecause of tracking, it is possible to suppress the displacement of thespot not only in the axis direction but also in the disk track directionto a minimum. Hence, the optical pickup lens device according to thefirst embodiment can suppress the risk of causing off-track errors.Namely, the optical pickup lens device according to the first embodimentcan suppress the amount of displacement of the spot, which ismagnification chromatic aberration, while correcting large axialchromatic aberration occurring in the objective lens due to a shortwavelength region and a high numerical aperture NA.

Second Embodiment

An optical pickup lens device according to a second embodiment will bedescribed below. As conventional techniques of correcting chromaticaberration in a lens device of an optical pickup device, JapaneseLaid-Open Patent Publications No. 7-294707 and No. 11-337818 propose anobjective lens which performs chromatic aberration correction using adiffractive lens structure having a multitude of zones formedconcentrically. Japanese Laid-Open Patent Publications No. 7-294707 andNo. 11-337818 propose the production of an objective lens by aninjection molding method using a resin material for the purpose offorming a diffractive lens structure in an objective lens with highaccuracy and at a low cost.

An objective lens described in Japanese Laid-Open Patent Publication No.7-294707 is supposed to be used for a bundle of rays with a wavelengthof 780 nm. An objective lens described in Japanese Laid-Open PatentPublication No.11-337818 is supposed to be used for a bundle of rayswith a reference wavelength of 650 nm. If the objective lenses describedin Japanese Laid-Open Patent Publications No. 7-294707 and No. 11-337818are used for a bundle of rays with wavelengths in a short wavelengthregion, such as a bundle of rays with a reference wavelength of 420 nmor less, the number of zones required to obtain a sufficient chromaticaberration correction effect increases and the widths of the zonesdecrease. This is because, as described above, with an increase in thewavelength dependency of the refractive index of a lens material in theshort wavelength region, the amount of chromatic aberration to becorrected increases.

In the objective lens described in Japanese Laid-Open PatentPublications No. 7-294707 and No. 11-337818, if the number of zonesincreases and the widths of the zones decrease, the production of anobjective lens becomes very difficult. First, if the number of zonesincreases and the widths of the zones decrease, it is difficult to forma mold for injection molding which conforms to such a fine pattern. Evenif the formation of the mold is possible, it is difficult tosufficiently transfer a fine mold pattern because of the viscosity of aresin, or the like. Consequently, if the objective lenses described inJapanese Laid-Open Patent Publications No. 7-294707 and No. 11-337818are used for a bundle of rays with wavelengths in the short wavelengthregion, such as a bundle of rays with a reference wavelength of 420 nmor less, it is difficult to produce an objective lens according to thedesign values, and thus ends up providing only such an objective lensthat causes a considerable loss in the amount of light due to patterndeviations.

In view of the foregoing problems, an object of the second embodiment isto provide an optical pickup lens device which provides easy productionand high performance even if used for a bundle of rays with wavelengthsin the short wavelength region, such as a bundle of rays with areference wavelength of 420 nm or less, and an aberration correctingelement used in such a lens device. Another object of the secondembodiment is to provide an optical pickup device having theaforementioned lens device.

One of the aforementioned objects is achieved by an aberrationcorrecting element described below. An aberration correcting element forallowing a bundle of incident rays to be transmitted therethrough has adiffractive surface having an optical power to deflect a bundle of raysby diffraction; and a phase step surface disposed at a locationdifferent from that of the diffractive surface and including a pluralityof zone regions defined by concentric circles with the optical axis of abundle of rays being at the center, and phase steps each formed at aboundary portion between the regions. The phase steps each generates aphase difference of an integer multiple of 2π radians with respect tothe reference wavelength, between a bundle of rays transmitted throughdifferent regions.

Since the aberration correcting element according to the secondembodiment has the above-described structure, an element can be providedwhich does not cause spherical aberration with respect to a bundle ofrays with the reference wavelength but causes spherical aberration withrespect to a bundle of rays with wavelengths displaced from thereference wavelength. By allowing this spherical aberration andspherical aberration in the diffractive surface to synergistically worktogether, desired large spherical aberration can be generated withoutforming a multitude of zones on the diffractive surface and withoutreducing the widths of the zones.

Preferably, the phase steps each generates a phase difference of 2πradians with respect to the reference wavelength, between a bundle ofrays transmitted through different regions. Since the aberrationcorrecting element according to the second embodiment has theabove-described structure, without causing high-order aberration, onlythird-order spherical aberration in particular can be corrected.

Preferably, the width of the regions in a direction orthogonal to theoptical axis decreases as the distance from the optical axis increases.Since the aberration correcting element according to the secondembodiment has the above-described structure, it is possible to correctspherical aberration whose amount rapidly increases as the distance fromthe optical axis increases, which is caused in particular by a high NAobjective lens.

Preferably, the phase step surface is an aspherical surface in which theoptical surfaces of the regions are defined by different asphericalsurface definitional equations. Since the aberration correcting elementaccording to the second embodiment, in which different regions havedifferent optimal aspherical surfaces, has the above-describedstructure, different regions can have different optimal asphericalsurfaces, and thus spherical aberration at the reference wavelength canbe corrected solely by the aberration correction element.

Preferably, the aberration correcting element includes a lens elementhaving a diffractive surface; and a lens element having a phase stepsurface. For example, the aberration correcting element may be composedof a single lens element with one surface having formed thereon adiffractive surface and the other surface having formed thereon a phasestep surface. Since the aberration correcting element according to thesecond embodiment has the above-described structure, molding andassembly and adjusting upon production are facilitated, and interfacialreflection occurring at a boundary surface is prevented from occurring.

One of the aforementioned objects is achieved by a lens device describedbelow. A lens device is used in an optical pickup device which performsat least one of reading, writing, and erasing of information byconverging a bundle of rays emitted from a light source onto an opticalinformation recording medium to form a spot, includes, in the order fromthe light source side to the optical information recording medium side,an aberration correcting element for allowing a bundle of rays emittedfrom a light source to be transmitted therethrough; and an objectivelens system for converging a bundle of rays coming from the aberrationcorrecting element onto an information recording medium to form a spot.The aberration correcting element has a diffractive surface having anoptical power to deflect a bundle of rays by diffraction; and a phasestep surface disposed at a location different from that of thediffractive surface and including a plurality of zone regions defined byconcentric circles with the optical axis of a bundle of rays being atthe center, and phase steps each formed at a boundary portion betweenthe regions. The phase steps each generates a phase difference of aninteger multiple of 2π radians with respect to the reference wavelength,between a bundle of rays transmitted through different regions.

Since the lens device according to the second embodiment has theabove-described structure, even if the oscillation wavelength isdisplaced from the reference wavelength due to large individualvariations in a semiconductor laser used as a light source or a changein oscillation wavelength caused by temperature changes, it is possibleto favorably form a spot by converging a bundle of rays onto an opticalinformation recording medium.

Preferably, the lens device is used for a bundle of rays with areference wavelength of 420 nm or less. Alternatively, preferably, thelens device is used for a bundle of rays having wavelengths in a rangewithin several nanometers of the reference wavelength.

An optical pickup device for performing at least one of reading,writing, and erasing of information by converging a bundle of rays ontoan optical information recording medium to form a spot, includes a lightsource for emitting a bundle of rays; a light converging section forconverging the bundle of rays emitted from the light source to form aspot on an optical information recording medium; a separation sectionfor separating a bundle of rays reflected by the optical informationrecording medium from an optical path of a bundle of rays from the lightsource to the light converging section; and a light receiving sectionfor receiving the bundle of rays separated by the separation section.The light converging section includes a lens device having an aberrationcorrecting element for allowing a bundle of rays emitted from the lightsource to be transmitted therethrough; and an objective lens system forconverging a bundle of rays coming from the aberration correctingelement onto an information recording medium to form a spot. Theaberration correcting element has a diffractive surface having anoptical power to deflect a bundle of rays by diffraction; and a phasestep surface disposed at a location different from that of thediffractive surface and including a plurality of zone regions defined byconcentric circles with the optical axis of a bundle of rays being atthe center, and phase steps each formed at a boundary portion betweenthe regions. The phase steps each generates a phase difference of aninteger multiple of 2π radians with respect to the reference wavelength,between a bundle of rays transmitted through different regions.

Since the optical pickup device according to the second embodiment hasthe above-described structure, even if the oscillation wavelength isdisplaced from the reference wavelength due to large individualvariations in a semiconductor laser used as a light source or a changein oscillation wavelength caused by temperature changes, it is possibleto favorably record information on an optical information recordingmedium, erase information from the optical information recording medium,or read information from the optical information recording medium,without causing tracking errors.

According to the second embodiment, it is possible to provide an opticalpickup lens device which provides easy production and high performanceeven if used for a bundle of rays with wavelengths in the shortwavelength region, such as a bundle of rays with a reference wavelengthof 420 nm or less, and an aberration correcting element used in such alens device. In addition, according to the second embodiment, it ispossible to provide an optical pickup device having the aforementionedlens device. The second embodiment will be described below withreference to the drawings.

FIG. 7 is a schematic structure diagram of an optical pickup deviceaccording to the second embodiment. An optical pickup device 30according to the second embodiment includes a light source section LS, alight converging section CO, a separation section SP, and a lightreceiving section RE. The light source section LS is composed of asemiconductor laser 26. The semiconductor laser 26 emits a leaser lightwith a reference wavelength of 410 nm. The light converging section COis composed of a collimating lens 24 and a lens device 21. Thecollimating lens 24 is a cemented lens formed by cementing two lenselements together. The lens device 21 includes an aberration correctingelement 22 and an objective lens 23. The structure of the lens device 21will be described later. The separation section SP is composed of a beamsplitter 25. The beam splitter 25 is formed by cementing together twoprisms in the shape of a triangular prism with a bottom surface in theshape of a right isosceles triangle, and has an optical film, on acemented surface, which has functionality to allow a certain proportionof a bundle of rays to be transmitted therethrough and reflect the restof the bundle of rays.

The light receiving section RE includes a detection lens 27 and a lightreceiving element 28. The light receiving element 28 is a photodiodewhich converts a bundle of incident rays into an electrical signalaccording to the intensity. A plate-like member disposed on a side ofthe objective lens 23 which is not adjacent to the aberration correctingelement 22 indicates part of an information recording medium 29 on whichthe optical pickup device 30 performs recording, reproduction, orerasing of information. A bundle of rays are converged onto an opticalrecording medium 9. There are shown an information recording surface 29a and a protective portion being present more on the light source sidethan the information recording surface 29 a and being transparent withrespect to a bundle of rays from the light source, and an illustrationof a structure corresponding to a substrate is omitted.

In FIG. 7, a bundle of rays coming from the semiconductor laser 26 aretransmitted through the beam splitter 25, and made into substantiallyparallel rays by the collimating lens 24 composed of a cemented lens andthen come out. The bundle of rays made into substantially parallel raysare transmitted through the aberration correcting element 22 and thenconverged, as a spot, onto the information recording surface 29 a of theinformation recording medium 29 by the objective lens 23.

The bundle of rays converged as a spot are reflected by pits withdifferent reflectivities formed on the information recording surface 29a. The bundle of rays reflected by the pits formed on the informationrecording surface 29 a are transmitted through the objective lens 23,the aberration correcting element 22, and the collimating lens 24 inthis order and then reach the beam splitter 5. The bundle of rays arereflected by the beam splitter 25 and then transmitted through thedetection lens 27. Further, the bundle of rays form a spot on a lightreceiving surface of the light receiving element 28 disposed at a lightconvergence position which is adjusted by the detection lens 27. Thelight receiving element 28 converts the change in the amount of thebundle of rays modulated by the information recording surface 29 a intoan electrical signal. The optical pickup device reads data stored on theoptical information recording medium, using the electrical signaloutputted from the light receiving element 28.

FIG. 8 is a schematic structure diagram showing a lens device used in anoptical pickup device according to the second embodiment. An aberrationcorrecting element 22 includes, in the order from the light source side,a diffractive surface S1 and a phase step surface S3, and is a lens madeof a resin. An objective lens 23 has a refractive surface S4 on thelight source side and a refractive surface S5 on the optical informationrecording medium side.

The diffractive surface S1 functions as an optical surface with apositive power for generating diffraction rays from incident raysentering the surface, and then converging the diffraction rays. Thediffractive surface S1 has a diffraction efficiency set such that theamount of +first-order diffraction rays becomes maximum. The phase stepsurface S3 functions as an optical surface with a negative power withrespect to diffraction rays, and has a power having an absolute valueequal to the absolute value of the positive power of the diffractivesurface S1. Consequently, the aberration correcting element 22 has nopower with respect to a bundle of rays with the reference wavelength,and if a bundle of parallel rays enters the aberration correctingelement 22, the aberration correcting element 22 allows the bundle ofparallel rays to come out as a bundle of parallel rays.

FIG. 9 is a schematic diagram showing the structure of a phase stepsurface of an aberration correcting element of a lens device used in anoptical pickup device according to the second embodiment. A phase stepsurface S3 includes a plurality of zone regions defined by concentriccircles with the optical axis of a bundle of rays being at the center,and phase steps each formed at a boundary portion between the regions.In FIG. 9, the center which includes the optical axis is referred to asregion 1, and the radius of the region 1 is referred to as H1. In thefollowing, the zone regions formed from the optical axis toward theperiphery are referred to as region 2, region 3, . . . and region n, inthe order from the optical axis side. The outside diameter of the region2 is referred to as H2, the outside diameter of the region 3 to H3, andthe outside diameter of the region n to Hn. The magnitude of a stepbetween the regions 1 and 2 in a direction along the optical axis isreferred to as A1, the magnitude of a step between the regions 2 and 3in the direction along the optical axis is referred to as A2, and themagnitude of a step between the regions n-1 and n in the direction alongthe optical axis is referred to as An.

The aberration correcting element 22 used in the lens device accordingto the second embodiment has five zone regions. Boundary portionsbetween the regions are configured such that the magnitude of theboundary portions in the direction along the optical axis increases byan integer multiple of λ₀/(n₀−1) (which is q in the present embodiment),where λ₀ is the reference wavelength of the semiconductor laser 26entering the lens device, and no is the refractive index of a resinmaterial of the aberration correcting element 22 with respect to lightwith wavelength λ₀.

The value of q is equal to the phase of 2π radians of the referencewavelength of the semiconductor laser 26. Consequently, the phasedifference between two different rays transmitted through the phase stepsurface S3 becomes an integer multiple of 2π, and thus the phase stepsurface S3 does not change spherical aberration in a bundle of raystransmitted through the phase step surface S3. The objective lens 23performs aberration correction on the reference wavelength; therefore,when a bundle of rays with the reference wavelength enter the objectivelens 23, the objective lens 23 forms a favorable spot on the informationrecording surface 29 a of the information recording medium 29.

Now, the case is considered where the wavelength at which thesemiconductor laser 26 oscillates is changed by several nanometers fromthe reference wavelength due to individual variations between theelements, temperature changes, or the like. Here, the oscillationwavelength of the semiconductor laser 26 displaced from the referencewavelength is denoted by λ₁, and the refractive index of the resinmaterial with respect to wavelength λ₁ is denoted by n₁. Under theseconditions, the phase difference between two different rays transmittedthrough the phase step surface S3 can be expressed by2πqλ₀(n₁−1)/((n₀−1)λ₁). Since this value deviates from an integermultiple of 2π with respect to the wavelength changed by severalnanometers from the reference wavelength, a bundle of rays transmittedthrough the phase step surface S3 generates spherical aberration.

Spherical aberration occurring in the phase step surface S3 can beadjusted by how the radius of each region formed on the phase stepsurface S3 is set with respect to the effective aperture of the opticalsystem, and what shape the surface of each region takes. Therefore, ifthe wavelength of a bundle of rays at which the semiconductor laser 26oscillates is displaced by several nanometers from the referencewavelength, the tendencies of spherical aberration occurring in thediffractive surface S1 and spherical aberration occurring in the phasestep surface S3 can be designed to be in the same direction and to be inthe opposite direction of spherical aberration generated by theobjective lens 23. By thus designing, it becomes possible to generatelarge aberration solely by the aberration correcting element 22 withoutforming a multitude of zones on a peripheral portion of the diffractivesurface S1 and without reducing the widths of the zones, as in the casewhere spherical aberration is generated only by the diffractive surfaceS1. By compensating spherical aberration between the aberrationcorrecting element 22 and the objective lens 23, a substantial imagepoint position on the optical axis can be corrected, and consequently,axial chromatic aberration can be corrected.

By making both surfaces of the aberration correcting element 22 to bediffractive surfaces, too, it is possible to generate large sphericalaberration without forming a multitude of zones on the diffractivesurface and without reducing the widths of the zones. However, if bothsurfaces are made to be diffractive surfaces, among a bundle of raystransmitted through the aberration correcting element, a loss in theamount of light used to form a spot on the optical information recordingsurface increases and thus it is not desirable. Hence, as in the lensdevice according to the embodiment, it is desirable that one surface ofthe aberration correcting element 22 be made to be a diffractive surfaceand the other surface be made to be a phase step surface.

Note that although the magnitude of the steps between regions formed onthe phase step surface S3 in a direction along the optical axis is setsuch that a phase difference of an integer multiple of 2π radians occursbetween a bundle of rays with the reference wavelength transmittedthrough different regions, the value of the integer can be appropriatelyset according to desired characteristics. For example, if the value isset so as to provide a phase difference corresponding to 2π radians, theamount of displacement from 2π of a phase difference occurring upondisplacement from the reference wavelength by several nanometersdecreases. Therefore, to take a large amount of aberration correction,it is necessary to deepen the steps to increase a phase difference.

By contrast, if the depth of the phase steps is increased, unnecessaryhigh-order spherical aberration increases, degrading the entireaberration; thus, if only third-order spherical aberration needs to becorrected, it is desirable that the depth of the phase steps be set to avalue corresponding to a phase difference of 2π which is the minimumnecessary.

Note that although the aberration correcting element 22 described in thesecond embodiment is a lens element into which the diffractive surfaceS1 and the phase step surface S3 are integrally formed, the aberrationcorrecting element 22 is not limited thereto. For example, theaberration correcting element 22 may be a combination of a lens elementhaving only a diffractive surface and a lens element having only a phasestep surface. However, taking into consideration molding and assemblyand adjusting upon production and interfacial reflection occurring at aboundary surface, it is desirable that the aberration correcting element22 be composed of an integrally-formed single lens element.

In addition, it is desirable that the aberration correcting element 22and the objective lens 23 be integrally held together and integrallymovable by an actuator.

It is desirable that the aberration correcting element 22 have regionswhose width in a direction orthogonal to the optical axis decreases asthe distance from the optical axis increases. In the case where theobjective lens is composed of a single lens element and is used at ahigh NA such as an NA of 0.8, the objective lens has characteristic thatthe amount of spherical aberration occurring upon displacement from thereference wavelength by several nanometers abruptly increases as thedistance from the optical axis in a direction orthogonal to the opticalaxis increases. To correct this characteristic, it is also necessary toincrease the amount of spherical aberration caused in the aberrationcorrecting element 22 toward the periphery. Therefore, it is desirablethat, since the number of phase steps also needs to be increased towardthe periphery, the width of the regions in the direction orthogonal tothe optical axis decrease toward the periphery.

In the case where the optical surfaces of the regions are connected toeach other with phase steps, the phase step surface may be an asphericalsurface defined by a single aspherical surface definitional equation ormay be an aspherical surface defined by different aspherical surfacedefinitional equations. Note, however, that it is desirable that thephase step surface be an aspherical surface defined by differentaspherical surface definitional equations for the following reason. Ascompared with an aberration correcting element having regions defined bya single aspherical surface definitional equation, an aberrationcorrecting element having different optimal aspherical surfaces fordifferent regions is able to correct spherical aberration at thereference wavelength solely by itself.

Specifically, in an aberration correcting element having regions definedby a single aspherical surface definitional equation, since the regionshave different thicknesses in the optical axis direction, sphericalaberration or an optical power component is generated even at thereference wavelength. On the other hand, in an aberration correctingelement having different optimal aspherical surfaces for differentregions, since the regions can be individually designed so as to preventspherical aberration or an optical power component from being generated,it is possible to further improve the characteristics of a lens device.

The aberration correcting element 22 is most effective when thereference wavelength of the semiconductor laser 26 is 420 nm or less.Generally, in a short wavelength region in which the wavelength is 420nm or less, since the dispersion of optical materials such as glass isvery large, there is a tendency that axial chromatic aberration in theoptical system also becomes very large. Hence, if the wavelength of thesemiconductor laser is changed even slightly, large axial chromaticaberration may occur. If large axial chromatic aberration occurs,tracking may not be performed, and as a result, stable recording,erasing, and reproduction cannot be performed. If the aberrationcorrecting element 22 is provided to a lens device used in such awavelength region, even if the reference wavelength of the semiconductorlaser 26 is changed, because axial chromatic aberration is corrected,stable tracking can be performed.

In the aberration correcting element 22, by appropriately designing thediffractive surface S1 or the phase step surface S3, it is possible toadjust spherical aberration occurring in the aberration correctingelement and correct axial chromatic aberration caused by a lens devicewhich can be used in the optical system other than an objective lens(e.g., the collimating lens 24, a protective layer provided on theinformation recording surface 29, etc.).

Although in the aberration correcting element 22 the order ofdiffraction to be designed is +first order, generally, any of ±m-thorder (m: integer) may be used. Although in the lens device of theembodiment the objective lens 2 is described to be composed of a singlelens, it may be composed of a plurality of combined lenses.

Although, in the lens device of the second embodiment, a bundle ofparallel rays enter the aberration correcting element 22, a bundle ofnon-parallel rays may enter the aberration correcting element 22.Further, although in the lens device of the second embodiment a bundleof parallel rays is present between the aberration correcting element 22and the objective lens 23, a bundle of non-parallel rays may be present.Although the aberration correcting element 22 has a diffractive surfacedisposed on the light source side and has a phase step surface disposedon the optical information recording medium side, it is also possible todispose them the other way around such that the phase step surface is onthe light source side and the diffractive surface is on the opticalinformation recording medium side.

As described above, the aberration correcting element according to thesecond embodiment can generate spherical aberration with respect to abundle of rays having wavelengths displaced from the referencewavelength, while not generating spherical aberration with respect to abundle of rays having the reference wavelength. By allowing thisspherical aberration and spherical aberration in the diffractive surfaceto synergistically work together, desired large spherical aberration canbe generated without forming a multitude of zones on the diffractivesurface and without reducing the widths of the zones. Further, theaberration correcting element according to the embodiment can beproduced using a resin, which enables easy production.

By applying this aberration correcting element to a lens device, even ifthe oscillation wavelength is displaced from the reference wavelengthdue to large individual variations in a semiconductor laser used as alight source or a change in oscillation wavelength caused by temperaturechanges, it is possible to favorably form a spot by converging a bundleof rays onto an optical information recording medium.

Further, by applying this lens device to an optical pickup device, evenif the oscillation wavelength is displaced from the reference wavelengthdue to large individual variations in a semiconductor laser used as alight source or a change in oscillation wavelength caused by temperaturechanges, it is possible to favorably record information on an opticalinformation recording medium, erase information from the opticalinformation recording medium, or read information from the opticalinformation recording medium, without causing tracking errors.

SECOND NUMERICAL EXAMPLE

Regarding a lens device according to the second embodiment, a specificnumerical example will be described below. An aberration correctingelement 22 is designed such that for a design wavelength the referencewavelength is 410 nm. In addition, for a bundle of rays, a bundle ofparallel rays are assumed to enter the aberration correcting element 22,the diameter of the bundle of parallel rays on the outgoing side is setto 2.21 mm at an incident surface of an objective lens 23. A phasegrating formed on a diffractive surface is represented by an ultra-highrefractive index method (for the ultra-high refractive index method, seeWilliam C. Sweatt: Describing holographic optical elements as lenses:Journal of Optical Society of America, Vol. 67, No. 6, June 1977).

The aspherical shape is given by the following (Eq. 2): $\begin{matrix}{X = {\frac{C_{j}h^{2}}{1 + \sqrt{1 - {\left( {1 + k_{j}} \right)C_{j}^{2}h^{2}}}} + {\Sigma\quad A_{j,n}h^{n}}}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$

The meaning of each symbol is as follows:

-   -   X: distance of a point on an aspherical surface whose height        from the optical axis is h, from a plane tangent to the vertex        of the aspherical surface,    -   h: height from the optical axis,    -   Cj: curvature of the j-th surface of a lens at the vertex of the        aspherical surface    -   (Cj=1/Rj: Rj is the radius of curvature of the j-th surface),    -   Kj: conic constant of the j-th surface of the lens, and    -   Aj, n: n-th order aspherical coefficient of the j-th surface of        the lens (j=3, 4, 5).

Table 6 provides numeric data about the lens device and opticalinformation recording medium of the second numerical example. In Table6, rj represents the radius of curvature of the j-th surface, djrepresents the j-th axial distance between surfaces, n410 represents therefractive index of the medium with respect to a wavelength of 410 nm,and ν represents the Abbe constant. TABLE 6 Sur- No. face r d n410 νAberration 1 s1 100000.000 0.000 4101.3141 −3.45 correcting 2 s2 Plane1.000 1.52256 56.4 element 3 s3 12.402000 2.000 Objective 4 s4 1.0899511.90681 1.77717 45.6 lens 5 s5 −3.138721 0.2428 Optical 6 s6 Plane 0.1001.61580 56.4 infor- 7 s7 Plane mation recording medium

In the second numerical example, a surface of the aberration correctingelement 22 on the light source side is made to be a diffractive surfaceS1 and a surface on the outgoing side, i.e., on the side of theobjective lens 23, is made to be a phase step surface S3, and axialchromatic aberration in the objective lens 23 is corrected. In addition,in the second numerical example, the absolute value of the diffractionpower of the diffractive surface S1 of the aberration correcting element22 on the light source side and the refractive power of the phase stepsurface S3 on the objective lens side are made to be negative, and theabsolute values are made to be the same, whereby the total optical powerof the aberration correcting element 22 is made to be 0. Further, in thesecond numerical example, the lens device is designed such thatfirst-order diffraction rays have the maximum amount of diffractionrays.

Table 7 shows numeric values indicating aspherical coefficients of athird surface S3, a fourth surface S4, and a fifth surface S5 of thesecond numerical example (see Table 6). Note that the third surface S3is a surface defined by a single aspherical surface definitionalequation when the surfaces are connected to each other with phase steps.Table 8 shows numeric values for the phase step surface S3 formed on theaberration correcting element 22. TABLE 7 3rd surface 4th surface 5thsurface k3 −0.8360125 k4 32.3466 k5 0 A3, 4 0.035208012 A4, 4 0.24439222A5, 4 −0.000020379 A3, 6 0.035187945 A4, 6 14.387668 A5, 6   3.47975E−08A3, 8 −0.091241412 A4, 8 −164.43004 A5, 8 −1.23535E−07 A3, 10 0.14573377A4, 722.13658 A5,    3.3379E−07 10 10 A3, 12 −0.077937629 A4, −730.2829812 A3, 14 −0.015357262 A4, −3352.4269 14 A3, 16 0.01649441 A4, 7626.125616

TABLE 8 Difference with respect to thickness Radius (Step depth) Region(mm) Phase step (mm) 1 0.60 — — 2 0.78 A2 0.000785 3 0.85 A3 0.000785 40.94 A4 0.000785 5 1.00 A5 0.000785

As shown in Table 8, the phase step surface S3 has the same magnitude inthe optical axis direction in regions 1 to 5 from optical between axis Psuch that all of the steps (difference in thickness) the regions 1 and2, between the regions 2 and 3, between the regions 3 and 4, and betweenthe regions 4 and 5 provide a phase difference of 2π with respect to awavelength of 410 nm.

FIG. 10 is a graph showing spherical aberration in the lens device ofthe second numerical example at wavelengths of 410 nm ±10 nm. In thedrawing, the horizontal axis represents the length in the optical axisdirection, and an axial image point for the case where the referencewavelength is 410 nm is taken as the point of origin. In addition, inthe drawing, the vertical axis represents the radius of a bundle ofparallel rays entering the aberration correcting element 22 and isnormalized by an effective aperture. In FIG. 10, axial chromaticaberration corresponds to a distance between curves for the wavelengthson the horizontal axis. As can be verified from FIG. 10, in the lensdevice of the second numerical example, the displacement of a focusposition in the optical axis direction shows almost no movement,regardless of wavelength.

The lens device of the second numerical example is compared with a lensdevice (comparative example) having the same conditions as the lensdevice of the second numerical example except for having no phase steps;the amount of defocus occurring at a focal point of the objective lensof the lens device of the second numerical example is reduced by about 4mλ per 1-nm wavelength change in the neighborhood of 410 nm, and theamount of displacement of the focal point in the optical axis directioncaused by wavelength changes is reduced by 0.013 μm per 1-nm change.

In the second numerical example, the amount of defocus and the amount ofreduction in axial chromatic aberration can be further increased byincreasing the number of regions in a phase step structure or byincreasing the step depth so that the amount of phase difference is amultiple of two, three, . . . of 2π. That is, in an element having adiffraction structure in which the amount of aberration correction isthe same, by providing phase steps, the amount of chromatic aberrationcorrection can be increased.

THIRD NUMERICAL EXAMPLE

A third numerical example uses the same structure as that of the secondnumerical example except for phase steps; as shown in Table 9, the stepdepth is set so as to provide a phase difference of an integer multipleof 2π with respect to a wavelength of 410 nm. Therefore, in the thirdnumerical example, the steps do not have a uniform depth. TABLE 9Difference with respect to thickness Radius (Step depth) Region (mm)Phase step (mm) 1 0.60 — — 2 0.78 A2 0.006280 3 0.85 A3 0.005495 4 0.94A4 0.003925 5 1.00 A5 0.003925

In a single objective lens 3, the NA is 0.8 or more, and the amount ofspherical aberration occurring upon displacement from the referencewavelength by several nanometers increases rapidly toward the periphery.Therefore, it is also necessary to increase the amount of sphericalaberration, which is caused to correct axial chromatic aberration,toward the periphery.

A lens device using an aberration correcting element 6 of the thirdnumerical example is compared with a lens device (comparative example)having the same conditions as the lens device of the third numericalexample except for having no phase steps; the amount of defocusoccurring at a focal point of the objective lens of the device of thethird numerical example is reduced by about 27 mλ per 1-nm wavelengthchange in the neighborhood of 410 nm, and the amount of displacement ofthe focal point in the optical axis direction caused by wavelengthchanges is reduced by 0.078 μm per 1-nm change.

The third numerical example describes that axial chromatic aberrationcan be favorably corrected by providing phase steps; however, sincehigh-order spherical aberration also increases because of the phasesteps, the total aberration is reduced to only about half its originalvalue. To reduce the entire aberration, the number of regions may beincreased. In particular, to reduce the change in third-order sphericalaberration, a phase difference occurring in the aberration correctingelement 22 may be increased.

Another Embodiment

The phase steps described in the second embodiment may be formed on theaberration correcting element of the first embodiment described above.By forming phase steps on the aberration correcting element of the firstembodiment, a higher-performance optical pickup lens device can beprovided.

INDUSTRIAL APPLICABILITY

The present invention is applicable to information recording andreproducing devices, etc., which perform writing, reproduction, orerasing on an information recording medium, such as a CD-ROM, a CD-R, aCD-RW, a DVD-ROM, a DVD−R, a DVD+R, a DVD−RW, a DVD+RW, an HD-DVD, or aBlu-Ray Disk. In particular, the present invention is suitable for usein information recording and reproduction devices, etc., which performwriting, reproduction, or erasing on a high-density recordableinformation recording medium which uses a bundle of rays with awavelength of 420 nm or less, such as an HD-DVD or a Blu-Ray Disk whichis the next generation DVD.

1. An optical pickup lens device used in an optical pickup device whichperforms at least one of reading, writing, and erasing of information byconverging onto an information recording medium a bundle of rays with awavelength range of 390 nm to 420 nm emitted from a light source, toform a spot, the optical pickup lens device comprising, in an order froma side of the light source: collimating means for converting the bundleof rays into parallel rays or predetermined convergent or divergentrays, the collimating means being movably held along a direction of anoptical axis of the bundle of rays emitted from the light source; anaberration correcting element for allowing a bundle of rays emitted fromthe collimating means to be transmitted therethrough; and an objectivelens element having a numerical aperture of 0.8 or more, and converginga bundle of rays coming from the aberration correcting element onto theinformation recording medium to form a spot, wherein the aberrationcorrecting element and the objective lens element are integrally heldtogether in a direction orthogonal to the optical axis so as to performtracking on the information recording medium, and the optical pickuplens device satisfies the following conditions:−0.1≦CAt≦−0.1  (1);−20≦CAf≦20  (2);−20≦CAm≦0  (3);−0.25≦θf≦0.25  (4); and−0.75≦θm≦0.75  (5), where CAt: axial chromatic aberration (μm/nm) in anentire optical system, CAf: axial chromatic aberration (μm/nm) in thecollimating means, CAm: axial chromatic aberration (μm/nm) in theaberration correcting element, θf: amount of change in an angle of abundle of outgoing rays from the collimating means per unit wavelength(min/nm), and θm: amount of change in an angle of a bundle of outgoingrays from the aberration correcting element per unit wavelength(min/nm).
 2. The optical pickup lens device according to claim 1,wherein the aberration correcting element is a diffractive lens providedseparately from the objective lens element and having an optical powerto deflect a bundle of rays by diffraction.
 3. The optical pickup lensdevice according to claim 1, wherein the aberration correcting elementis an element provided separately from the objective lens element, andhas a phase step surface including a plurality of zone regions definedby concentric circles with the optical axis being at a center; and phasesteps each formed at a boundary portion between the regions.
 4. Anoptical pickup device which performs at least one of reading, writing,and erasing of information by converging a bundle of rays onto aninformation recording medium to form a spot, the optical pickup devicecomprising: a light source for emitting a bundle of rays with awavelength range of 390 nm to 420 nm; collimating means for convertingthe bundle of rays into parallel rays or predetermined convergent ordivergent rays, the collimating means being movably held along adirection of an optical axis of the bundle of rays emitted from thelight source; an aberration correcting element for allowing a bundle ofrays emitted from the collimating means to be transmitted therethrough;and an objective lens element having a numerical aperture of 0.8 ormore, and converging a bundle of rays coming from the aberrationcorrecting element onto the information recording medium to form a spot,wherein the aberration correcting element and the objective lens elementare integrally held together in a direction orthogonal to the opticalaxis so as to perform tracking on the information recording medium, andthe optical pickup device satisfies the following conditions:−0.1≦CAt≦−0.1  (1);−20≦CAf≦20  (2);−20≦CAm≦0  (3);−0.25≦θf≦0.25  (4); and−0.75≦θm≦0.75  (5), where CAt: axial chromatic aberration (μm/nm) in anentire optical system, CAf: axial chromatic aberration (μm/nm) in thecollimating means, CAm: axial chromatic aberration (μm/nm) in theaberration correcting element, θf: amount of change in an angle of abundle of outgoing rays from the collimating means per unit wavelength(min/nm), and θm: amount of change in an angle of a bundle of outgoingrays from the aberration correcting element per unit wavelength(min/nm).